The present invention relates to the imaging of a plate having a coating imageable by infrared radiation and more specifically to infrared imaging with simultaneous supplemental infrared heating to improve the imaging process. The invention is particularly directed to the imaging of lithographic printing plates.
Positive-working lithographic printing plates or other imageable plates which have a coating selectively removed from the plate by exposure to imaging infrared laser radiation are known in the prior art. One category of these plates has a coating on a substrate wherein the coating is transparent to the imaging infrared radiation and wherein the substrate absorbs the infrared radiation generating heat. The generated heat then removes the coating in the imaged areas by ablation or combustion of the coating. International Patent Application Publication No. WO 98/52743, published Nov. 26, 1998 and the related U.S. patent application Ser. No. 09/079,735, filed May 15, 1998, now U.S. Pat. No. 6,145,565, discloses a laser imageable printing plate having a coating which is transparent to the laser imaging radiation and having a substrate which absorbs the radiation. In that patent application, the coating is ablated from the substrate by the heat generated as a result of the substrate absorbing the radiation. The ablation is a function of the heat disrupting the physical bond between the substrate and the coating and pulverizing the coating probably by expansion of the substrate beneath the relatively brittle coating. Another known type of a coating which can be removed by heating the substrate with the infrared laser radiation is a coating which will undergo a thermally induced chemical reaction such as combustion. In either case, certain minimum laser power levels and exposure times are required to produce the heating required.